In membrane exchanger devices, the exchanger membranes are placed parallel to one another in the form of a stack and they are separated and supported by the separator frames. These frames comprise a rigid part which is impermeable to fluids and which has a central opening containing an expanded structure, e.g. a grid. The rigid part is called the seal plane and provides sealing in the device. It is pierced by orifices which allow the fluid to flow through the stack. The orifices constitute fluid inlet and outlet ducts. The expanded structure allows the fluid to flow through the central open portion.
Each exchange zone is constituted by a stack comprising a first exchange membrane which is permeable to some items, a separator frame, and a second exchanger membrane which is permeable to the same items or to other items. In order to ensure that exchange takes place under good conditions, it is necessary for there to be a large flow of fluid through each exchange zone.
One known way of increasing speed of flow is to reduce the thickness of the zone through which the fluid flows. However, since the membranes expand when in the presence of the fluid, it is necessary for the thickness of the seal plane of the separator frame to be greater than the thickness of the expanded structure so as to enable the fluid to flow through the exchange zone in spite of membrane expansion.
However, it is observed that the difference in thickness between the seal plane and the expanded structure gives rise to leaks at diffusers which are constituted by openings formed through the seal plane and which correspond to the fluid inlet and outlets ducts in the exchange zone. These leaks are due to membrane flexibility: the membranes hollow out over the diffusers and allow fluid to flow into a given exchange zone, said fluid coming from an inlet duct and being intended for delivery to the adjacent exchange zone in the stack. Such partial mixing of the fluids is naturally unacceptable.
Proposals have already been made to mitigate this drawback by increasing the thickness of the seal plane between the expanded structure and the structures of the flow ducts for the fluid that does not flow through the exchange zone in question. One such device is described in French patent number 2 189 091.
Proposals have also been made in U.S. Pat. No. 4,319,978 to mitigate this drawback by making a separator frame which comprises five layers: an inner layer; two intermediate layers placed on either side of the inner layer; and two outer layers placed on either side of the two intermediate layers. All of these layers have a central portion with an opening. Only the inner layer has the diffusers. The outer layers are compressible, for example they may be made of neoprene. The intermediate layers are sufficiently stiff to prevent the diffusers being reduced in area.
The present invention provides a novel design of an exchanger device for exchanging items between two fluids, said items being contained in at least one of the fluids, and mitigating the above-mentioned drawback.